Use _.isFunction so ClojureScript doesn't break underscore

Clojurescript adds a call method to strings, so passing a string to
_.invoke produces the wrong behavior.

underscore.js

@@ -218,7 +218,7 @@
   _.invoke = function(obj, method) {
     var args = slice.call(arguments, 2);
     return _.map(obj, function(value) {
-      return (method.call ? method || value : value[method]).apply(value, args);
+      return (_.isFunction(method) ? method || value : value[method]).apply(value, args);
     });
   };